This invention relates to flywheel energy conversion devices that include motor-generators and methods for providing increased output power, and more particularly toward flywheel energy conversion devices including brushless motor-generators having low inductance armature windings. The armature windings of the present invention are located in an air gap of an unusually high reluctance field circuit including large air gaps in place of traditional armature windings that are enclosed in the high permeability parts of a lower reluctance field circuit.
One area where flywheel energy storage devices may prove advantageous is in situations requiring a continuous supply of reserve power in the event of a primary power source failure (i.e., failure by a utility company supply). In such situations, it is often required that a secondary power source supply a nominal amount of power for a certain time period so that various pieces of equipment utilizing primary power may be shut down in a relatively normal fashion, rather than the instantaneous shut down that would occur from a loss of primary power without a backup supply. A traditional approach to resolving this problem is the use of a bank of chemical batteries, often combined with an emergency generator.
For example, in a paper mill, substantially liquid paper pulp is sprayed onto a rotating wire mesh and then carried through a long series of rollers through ovens to remove the moisture from the pulp. It may take several minutes for the liquid pulp to pass through all of the ovens before the pulp has dried and reached the end of the line where it is rolled up onto high speed spools. An instantaneous loss of power under such circumstances would be catastrophic. Therefore, paper mills often have one or more large rooms filled with chemical batteries to provide backup power to keep all of the equipment running while the pulp supply is shut off and the remainder of the pulp already on the production line is processed.
Chemical batteries, however, suffer from various deficiencies including bulkiness, lack of reliability, limited lifespan, high maintenance costs and relatively low safety. For example, chemical batteries require relatively constant and complex recharging, depending on the type of batteries involved to insure that the batteries continue to operate efficiently and maintain their full storage capacity. Additionally, chemical batteries raise various safety considerations due to the general nature of the large quantities of caustic chemicals involved. Typical large battery installations often require special venting and air-conditioning systems for the dedicated battery storage rooms.
In order to provide an efficient replacement for chemical batteries, flywheel energy storage devices must operate at high levels of energy conversion efficiency. Thus, flywheel devices are often designed to operate in a vacuum so as to minimize the energy losses due to air drag friction (e.g., see Benedetti et al. U.S. Pat. No. 4,444,444). The vacuum condition demands that heat generation in the rotating components be minimized because rotor heat in a vacuum can only be dissipated by radiation or conduction through bearing surfaces which are small and have limited heat conducting capacity. In addition, brushes used to transfer current between stationary components and rotating components in vacuum conditions are subject to more destructive arcing than brushes operating in air. This essentially limits the energy storage device to brushless operation because brushes tend to exhibit extremely short lifespans when operated in vacuum conditions. The use of brushless motor-generators in flywheel storage devices is complicated, however, by the fact that brushless motor-generators typically utilize heat generating components such as rotating rectifier assemblies and rotating coils, as described below.
The use of brushless generators is well known throughout various industries. For example, automobile manufacturers often utilize brushless generators to provide electrical power to vehicles. Prior brushless generators suffer from a variety of problems that make them poor candidates for use with flywheel energy storage devices. Many of these prior generators utilize bent-over teeth as magnetic fingers in the rotor assembly. For example, Godkin et al. U.S. Pat. No. 4,611,139 and Farr U.S. Pat. No. 4,654,551 both disclose brushless alternators that include magnetic bent-over fingers to produce varying magnetic flux in the stator core. The bent-over teeth in these devices are simply inappropriate for flywheel applications because the high speeds at which the tip of the flywheel must rotate would cause high stress concentrations at the bend in the teeth which severely compromise operational safety. To maintain safe operations in view of the high stress concentrations, known flywheel devices often operate at lower rotational speeds that, unfortunately, result in less stored energy for a given volume.
Another kind of brushless generator operates by applying a small input signal to an exciter winding that induces a much larger signal in a rotating member. The input signal, which may be a DC current or a low frequency AC current, causes an AC current to be induced in the rotating member. The AC current is then converted to DC by a rectifier assembly typically located within the rotating member, as is known in the art (e.g., see Pinchott U.S. Pat. No. 5,065,484). The rectified DC current flows through the main windings (on the rotating member) and creates a large rotating magnetic field. The rotating field interacts with the main armature to generate a large AC signal in the armature windings. This large AC signal, which is delivered to the external load, may be effectively 10,000 times greater than the signal that was input to the exciter.
In some instances, the exciter may itself be excited by a permanent magnet generator (PMG). One known example of an alternator which utilizes PMGs is described in Farr U.S. Pat. No. 4,654,551. Farr's magnetic flux field is generated by a rotating permanent magnet ring and a toroidal control coil, where the toroidal control coil is mounted to add or subtract in the magnetic relationship with the ring. Farr, however, may experience potentially severe core losses due to the nature of the stationary iron core armature device.
As with most known electro-magnetic devices, many brushless generators are typically manufactured using iron cores in both the exciter and main armatures. For example, Giuffrida U.S. Pat. No. 4,647,806 describes a brushless alternator having an exciter armature formed from a laminated stack of steel plates, and Mallick et al. U.S. Pat. No. 4,385,251 describes an inductor-alternator having armature coils wound around slots cut into laminated stack stators. While both Giuffrida and Mallick described improvements over known devices at the time, both patents represent machines that, unfortunately, produce various energy losses (e.g., core losses) and have high armature inductances resulting in limited power density.
Another necessary consideration in designing flywheel devices relates to the negative effects of weight of the rotor. The weight of the rotor is particularly relevant in energy storage applications--flywheel rotors typically weigh hundreds of pounds--because the rotor must rotate at exceedingly high speeds in order to store kinetic energy. As such, the mechanical bearings supporting the rotor are often placed under high stress resulting in rapid bearing wear.
One known method for addressing bearing wear in flywheel applications is the replacement of the conventional bearings with magnetic bearings. For example, Benedetti et al. U.S. Pat. No. 4,444,444 describes a magnetically suspended flywheel that employs a double electromagnet and a servoloop for restoring equilibrium to a levitated rotating member. The electromagnets, which are attached to a stationary shaft, interact with permanent magnets and a mobile armature attached to the rotating member to provide a magnetic attraction "equal to the force of gravity" acting on the mass of the rotor. Such a solution is relatively complex, requiring the attachment of several additional components to the stationary and rotating parts of the device.
Additionally, applications such as Benedetti often require "air-core" armature coils because an iron core armature would cause magnetic instability by competing with the stabilizing magnetic forces of the magnetic bearing. However, such devices also require a very large volume of expensive permanent magnet material for the rotating member that is often structurally complex to implement (e.g., Benedetti's armature calls for twelve rotating magnets having successively opposite poles fixed about at least one of two rings). Further, implementations such as Benedetti essentially have limited output power due to physical considerations (Benedetti discusses a practical embodiment in which a 370 kg rotor provides up to 10 kw of power).
Another consideration that must be accounted for when implementing electrical machines is the negative effects of eddy currents in the unlaminated materials frequently used as part of the flux carrying magnetic circuit. For example, Mallick et al. U.S. Pat. No. 4,385,251 provides a flux shield in the form of, for example, a conducting ring concentric with the rotor, to help prevent time varying fluxes from inducing eddy currents in the rotor steel and unlaminated back iron because such eddy currents lead to performance losses in the machine. However, Mallick also notes that eddy currents are induced in the flux shields resulting in losses, but indicates that the losses are reduced when compared to machines without the flux shield.
In view of the foregoing, it is an object of this invention to provide an improved flywheel energy conversion device that efficiently provides high output power, including a compact design resulting in a high power density.
It is also an object of the present invention to provide an improved flywheel energy conversion device that includes a brushless generator for use in vacuum conditions, where a minimum of power is dissipated in the rotating frame.
It is a further object of the present invention to provide an improved flywheel energy conversion device that may be safely operated at substantially high rpm.
It is an additional object of the present invention to provide methods and apparatus for reducing the effects of core losses on high speed flywheel energy storage devices.
It is a still further object of the present invention to provide improved flywheel energy conversion devices that may be produced at low costs when compared to currently known technologies.